In many different communications applications, a common signal path is coupled both to the input of a receiver and to the output of a transmitter. For example, in a transceiver, such as a cellular or cordless telephone, an antenna may be coupled to the input of the receiver and to the output of the transmitter. In such an arrangement, a duplexer is used to couple the common signal path to the input of the receiver and to the output of the transmitter. The duplexer provides the necessary coupling while preventing the modulated transmit signal generated by the transmitter from being coupled from the antenna back to the input of the receiver and overloading the receiver.
One long-established form of duplexer is the half duplexer. Half duplexers are described by C. K. Campbell in SURFACE ACOUSTIC WAVE DEVICES FOR MOBILE AND WIRELESS COMMUNICATION., pp. 253-272, Academic Press, New York, (1998). A half duplexer uses a switch to connect the antenna to the receiver or the transmitter on a time division basis. The half duplexer has good coupling and attenuation properties, but is nevertheless an unacceptable solution for telephony applications because it does not allow both parties to speak (and be heard) simultaneously.
A more acceptable form of duplexer for telephony applications is the full duplexer, also described by Campbell. To enable a full duplexer to be used, the transmit signal must be at a different frequency from the receive signal. The full duplexer lacks a switch and incorporates band-pass filters that isolate the transmit signal from the receive signal according to the frequencies of the signals. FIG. 1 shows a conventional front-end circuit 10 such as that used in a cellular telephone, personal communication system (PCS) device or other transmit/receive apparatus. In this, the output of the power amplifier 12 of the transmitter 14 and the input of the low-noise amplifier 16 of the receiver 18 are connected to the duplexer 20, which is a full duplexer. Also connected to the duplexer is the antenna 22.
The duplexer 20 is a three-port device having a transmit port 24, a receive port 26 and an antenna port 28. The antenna port is connected to the transmit port through the band-pass filter 30 and to the receive port through the series arrangement of the 90.degree. phase shifter 34 and band-pass filter 32. The pass bands of the band-pass filters 30 and 32 are respectively centered on the frequency range of the transmit signal generated by the transmitter 14 and that of the receive signals to which the receiver 18 can be tuned. In the example shown, band-pass filters are configured such that the high-frequency stop band of the band-pass filter 30 overlaps the pass-band of the band-pass filter 32 and the low-frequency stop band of the band-pass filter 32 overlaps the pass-band of the band-pass filter 30.
The requirements for the band-pass filters 30 and 32 constituting the duplexer 20 are quite stringent. The band-pass filters isolate the very weak receive signal generated by the antenna 22 and fed to the input of the low-noise amplifier 16 from the strong transmit signal generated by the power amplifier 12. In a typical embodiment, the sensitivity of the low noise amplifier 16 is of the order of -100 dBm, and the power amplifier 12 can feed power levels of about 28 dBm into the duplexer. In such an example, the duplexer must attenuate the transmit signal by about 50 dB between the antenna port 28 and the receive port 26 to prevent the residual transmit signal mixed with the receive signal at the receive port from overloading the low-noise amplifier.
One type of mobile telephone that is becoming increasingly popular is the personal communication system (PCS) that uses Code Division Multiple Access (CDMA). CDMA PCS is described in T. S. Rapport, ed., CELLULAR RADIO & PERSONAL COMMUNICATIONS, VOL. 2, pp.501-509, IEEE Press, Piscataway, N.J., (1996). CDMA PCS devices operate in frequency bands at about 1,900 MHz and impose especially stringent requirements on the duplexer performance. The guard band between the portions of the spectrum assigned to the transmit signal and the receive signal is only about 1% of the carrier frequency, i.e., 20 MHz. The bandwidth of the portions of the spectrum assigned to the transmit signal and the receive signal are about 3% of the carrier frequency, i.e., 60 MHz. This means that the band-pass filters 30 and 32 are required to have an extremely sharp roll-off. FIG. 2 shows the basic arrangement of the transmit and receive bands. The required characteristics of the band-pass filters 30 and 32 are shown at 36 and 38, respectively.
Cellular telephones and PCS devices are constantly being made smaller and lower in cost. Several stacked printed circuit boards are typically used to accommodate the circuitry of the PCS device in the overall package size. Not only must the components mounted on the printed circuit boards be miniaturized, they must meet stringent height requirements. Components taller than the height limit require the printed circuit boards to be spaced further apart, which reduces the packing density that can be achieved. Alternatively, over-height components require that holes be cut in at least one adjacent printed circuit board to accommodate them, which both reduces the packing density and increases assembly costs.
Another challenge for the duplexer 20 is its power handling capability. The power amplifier 12 in the transmitter 14 can deliver up to 1 Watt of power to the transmit port 24 of the duplexer 20. Miniaturized as just described, the band-pass filters 30 and 32 must be capable of transmitting such power without being destroyed, or without its characteristics degrading with use.
Current-generation PCS devices use a ceramic filter as the duplexer 20. However, such ceramic filters are bulky, measuring some 28.times.8.times.5 mm, are over-height components and are expensive. Samples of such filters show evidence of having been individually tuned, which accounts for some of the cost of such devices.
Surface acoustic wave (SAW) filters have also been used as duplexers in cellular telephones and PCS devices, see, for example, O. Ikata, N. Nishihara, Y. Satoh, H. Fukushima and N. Hirisawa, A Design of Antenna Duplexer Using Ladder Type SAW Filters, PROC. 1998 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM, SENDAI, JAPAN, paper O-1 (Oct. 1998). The roll-off of a SAW filter with sufficient power handling capability is insufficiently steep for the CDMA application just described. Instead, two SAW filters and an electronic switch have to be used. One of the filters covers the upper half of the transmit and receive bands, the other covers the lower half of the transmit and receive bands. The electronic switch selects the appropriate filter depending on the portions of the transmit and receive bands in which the PCS device is operating. Thus, a duplexer based on SAW filters is also unacceptable bulky, complex, expensive and may be subject to failure in the event of a surge in the transmitter output power.
What is needed, then, is a duplexer that has sufficiently steep filter characteristics to enable it to be used in applications, such as CDMA PCS devices, in which the separation between the transmit and receive bands is only about 1% of the operating frequency and in which power levels exceeding one Watt do not impair the reliability of the duplexer or the long-term stability of the filter characteristics. The duplexer should be substantially smaller than current duplexers based on ceramic filters or SAW filters, and should not require individual tuning so that the cost of manufacture can be kept low.